THE FROG- HE ART 73 



reach the muscle always at the commencement of the period of relaxa- 

 tion, there is an appearance of fusion of the successive contractions ; 

 but they never overtop one another as in tetanus of skeletal muscle. 



3. " All or none " contraction. Put the secondary coil far from the 

 primary, and excite the preparation by breaking the primary circuit. 

 Determine the excitability by ascertaining to what division of the scale 

 the secondary coil must approach the primary before a contraction is 

 produced. This represents the liminal stimulus. Now bring the 

 secondary nearer the primary and again stimulate. The contraction is 

 riot appreciably larger. Repeat with stronger and stronger stimuli : 

 lastly, repeat with diminishing strength of stimulus. Apart from the 

 " staircase " effect already studied, in every case the extent of contraction 

 (if any) is the same (compare with skeletal muscle, p. 41). This 

 difference is, however, probably connected with the fact that the indi- 

 vidual fibres of skeletal muscle contract independently of one another, 

 and with the stronger stimuli a greater number are thrown into con- 

 traction. 



